Conventionally, electric vehicles mounted with an assembled battery device as a driving power source are known. One example of an assembled battery device has a plurality of assembled battery modules which are connected in series, and a battery management unit (BMU) to control the respective assembled battery modules. Each of the assembled battery modules is provided with an assembled battery having a plurality of secondary battery cells which are connected in series, and a voltage temperature monitoring circuit (VTM) to monitor the assembled battery. The battery management unit monitors a pack voltage and a pack current of a battery pack composed of a plurality of assembled battery modules, and cell voltages that are voltages of the respective secondary battery cells which the assembled battery module has, and so on.
When an assembled battery device is stored for a long time, the voltage balance between secondary battery cells may remarkably deviate by the difference of self-discharge amount between the secondary battery cells. A secondary battery cell with a large self-discharge amount has a small remaining capacity, and a secondary battery cell with a small self-discharge amount has a large remaining capacity. If an assembled battery device is discharged in the state in which the voltage balances between the respective secondary battery cells remarkably deviate, a secondary battery cell having the smallest remaining capacity, out of a plurality of the secondary battery cells, firstly reaches a working lower limit voltage. Similarly, if the assembled battery device is charged in the state in which the voltage balances between the respective secondary battery cells remarkably deviate, a secondary battery cell having the largest remaining capacity, out of the plurality of secondary battery cells, firstly reaches a working upper limit voltage. Accordingly, the effective capacity of the assembled battery device becomes smaller compared with the capacity (or, a rated capacity) before stored.